How does a vacuum tube diode convert alternating current into direct current?

A vacuum tube diode converts alternating current (AC) into direct current (DC) by acting as a one-way valve for electrons.1

The device consists of two electrodes housed within an evacuated glass or metal envelope: a cathode and an anode. Through a process called thermionic emission, the cathode is heated until it emits electrons from its surface.123

The conversion process works as follows:
* Forward Bias: When a positive voltage is applied to the anode, it attracts the electrons emitted by the heated cathode, allowing current to flow through the tube.1
* Reverse Bias: When the voltage polarity reverses and the anode becomes negative, it repels the electrons, preventing them from escaping the cathode. Consequently, no current flows.1

Because the diode only conducts when the anode is positive, it effectively blocks the negative portion of an AC signal, resulting in a rectified, pulsating direct current. This process is known as half-wave rectification.41